Configuration of heat transfer tubing for vapor condensation on its outer surface



De.2,1969 was, 3,481,394

J. G. CONFIGURATION OF HE TRANSFER'TUBING FOR VAPOR SATION CE CONDEN ON ITS OUTER SURFA Filed June 26, 1957 INVENT OR JAMES G.WITHERSJ WW,W

ATTORNEYS United States Patent US. Cl. 165-179 5 Claims I ABSTRACT OF THE DISCLOSURE Heat transfer tubing having external outwardly extending fin convolutions and internal inwardly extending rib convolutions, both preferably helical, the axial pitch of the rib convolutions being substantially greater than that of the fin convolutions.

BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTION Further development work relating to improvements in efficiency in heat transfer in tubing, particularly tubing designed for condensation on its exterior surface, and for heat transfer to a cooling liquid flowing through the tubing, has indicated the possibility of overall improvement if the internal and external surfaces are modified more or less independently. In other words, it has become apparent that the internal ribs in many cases should have a pitch or axial spacing substantially greater, and in some cases several times greater, than the pitch or axial spacing of external fins.

It is accordingly an object of the present invention to provide a heat transfer tube in which internal ribs are provided, preferably in a helical configuration, at a pitch or axial spacing which is substantially greater, and in some cases several times greater, than the pitch or axial spacing of external fins, preferably helical, which are formed on the exterior of the tubing.

More specifically, it is an object of the present invention to provide a heat transfer tube having a plurality of separate helically extending external fins provided in the manner of a multiple-start screw so. that each convolution of each fin is interposed between two convolutions of a different fin or fins so as to provide a plurality of separate external grooves, a single internal helical rib in alignment with only one of the external helical grooves.

Other objects and features of the invention will become apparent as the description proceeds, especially when w taken in conjunction with the accompanying drawing, illustrating preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING 3,481,394 Patented Dec. 2, 1969 FIGURE 5 is a view similar to FIGURE 4 illustrating another method and apparatus for forming the tubing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates particularly to improved configurations of tubing for the transfer of heat as required for condensation of vapor on the external surface of fluid cooled tubes. An example of such heat transfer application is that of steam condensing on watercooled tubes. Further examples might involve: vapor heating of viscous process fluids; or, in the case of cascade refrigeration, refrigerant vapor condensing on the external surface of tubing, with a second refrigerant being boiled as it flows through the tubing.

The utilization of specially designed tubing shapes and dimensions for steam condensing is disclosed in prior Rodgers Patent 3,217,779, assigned to assignee herein. In the Rodgers patent a variety of shapes is mentioned with optima depending on condensing conditions. In some cases it is advantageous to utilize partially or totally transverse fins, protuberances or the like on both the internal and external surfaces. Free draining of condensate from the external surface, combined with improved turbulence of the cooling liquid flowing through the tubing, results in reduced overall thermal resistance as compared with plain cylindrical tubing.

The prior Rodgers patent emphasized the importance of the ratio S/H in which S is the axial spacing between adjacent convolutions of the internal rib, and H is the radial height of the internal tab, on the internal heat trans fer performance. However, in the Rodgers patent the disclosure is of an arrangement in which the number and axial spacing between external fins and internal fins is identical. This results from the fact that the internal ribs are produced by a rolling operation displacing material from external grooves into the external fin configuration.

It has now been found that in many cases optimum overall performance requires substantially different patterns of external fins and internal ribs. Thus, in some cases it may be desirable to have the axial spacing or pitch of the internal ribs as much as five or six times as great as the pitch or axial spacing between adjacent convolu tions of the external fin. The factors which dictate the optimum spacing between ribs and between fins, as well as the relative height of the fins and ribs, are quite different and in fact, bear no relationship to each other.

In accordance with the present invention, tubes are formed which have an external multiple-start fin configuration consisting of a plurality of helically disposed continuous fins, the fins being arranged in the manner of a multiple-start screw so that in the case of a two-start fin for example, adjacent convolutions of fins throughout the length of the tube are portions of different fins.

Having an external fin configuration comprising a multiplicity of interleaved separate external continuous helical fins, there will be a corresponding number of separate distinct interleaved helical grooves. In accordance with the present invention, one of this multiplicity of grooves is deepened by an operation in which the material of the tube is displaced inwardly to produce a single continuous internal helical rib which accordingly extends opposite to and matches the external helical groove which was deepened to form the internal rib. With the foregoing arrangement, if a two-start external fin configuration is employed, the single internal rib will have a pitch or axial spacing between adjacent convolutions equal to twice the pitch or axial spacing between adjacent convolutions of the external fin. Similarly, if the external fin configuration has n starts, and only one of the resulting external grooves is deepened, the axial spacing or pitch between adjacent convolutions of the internal rib will be n times" as great as the pitch or axial spacing between adjacent convolu tions of the external fins.

In FIGURE 1 there is shown a fragmentary section of a tube having two starts producing two continuous helical fins one of which is designated at 12 and the other at 14. This results in the corresponding formation of two separate helical grooves 16 and 18. The groove 16 is allowed to remain in the condition existing after material has been displaced therefrom by rolling to form the adjacent fins 12 and 14. However, the remaining helical groove 18 is illustrated as having been deepened so as to produce an internally helically extending rib 19. It will be apparent by inspection that the axial spacing Si between adjacent convolutions of the internal rib is substantially double the pitch or axial spacing Se between adjacent fin convolutions at the exterior of the tube.

It may be mentioned at this time that the operation can be carried out so as to produce internal ribs 19 which have a height H as required to maintain the desired S/H ratio.

It may also be mentioned that while it is considered desirable to employ helical fins and ribs, the ribs and fins may if desired, be provided to extend straight around the tube so as to be circular or annular in shape.

In FIGURE 2 there is shown a modified tube 20 having a three-start fin construction providing external fins 21, 22 and 23. Similarly, this external arrangement will provide internal helical grooves 24, 25 and 26. With this arrangement only one of the grooves; namely, the groove 26, is deepened as illustrated as to provide the internal helically extending rib 27. Accordingly, the axial spac ing Si between adjacent rib convolutions in this case will be three times the axial spacing of the external fins.

In FIGURE 3 there is illustrated a tube 30 similar to the tubes previously described except that the fins here generally designated at 32, are the result of a six-start arrangement so that if one of the groves, as for example the groove 34, is deepened, it produces an internal rib 36 having a pitch or axial spacing Si which is six times as great as the axial spacing Se between adjacent fin convolutions.

In order to produce the tube as illustrated in FIG- URES 13, where the fin and rib configuration is helical, method and apparatus best illustrated in FIGURE 4 may be employed. In this figure the tube, here designated T, is advanced over a cylindrical mandrel 40 adapted to support the internal surface of the tube and to maintain it substantially smooth during the fin rolling operation. Associated with the tube are a plurality of fin forming rolls indicated generally at 42, only one of which is illustrated. The fin forming rolls comprise an arbor 44 on which a plurality of discs 46 are provided, the discs having a constant axial spacing and being shaped to be rolled into the material of the tubing to produce helical fins 48 and intermediate helical grooves 50. The arbor shafts are disposed at the appropriate angle to the axis of the arbor and tube so that each of the discs forms a separate helical groove 50. In other words, the fins produced by the apparatus illustrated in FIGURE 4 is a six-start helical fin providing a corresponding number of helical grooves 50. In this figure one of the discs 52 is substantially larger than the remaining discs 46 and is located beyond the end of the mandrel 40 so that the material of the tube opposite the rib forming disc 52 is unsupported. Accordingly, as the tube is advanced over the mandrel as a result of the rotation of the rolls 42, the smaller discs 46 form the fins 48 and the shallow grooves 50, whereas the disc 52 forms the relatively deeper groove 54 and cooperates in the formation of the fins 48 at both sides thereof.

It will be observed that in the illustrated embodiment only six discs are illustrated, five being the relatively small discs 46 and the remaining disc being the relatively large disc 52. With this arrangement and producing a six-start fin, the disc 52 is required to form the relatively deep groove 54 in its entirety. Alternatively, an additional small disc 46 could be provided at the left of the discs illustrated in the figure so that the function of the disc 52 would be merely to deepen a groove 50 previously formed by one of the small discs 46.

Referring now to FIGURE 5 there is illustrated apparatus for performing a somewhat different fin and rib forming operation on tubing. In this figure a rolling tool generally designated is provided comprising an arbor 62 on which a plurality of discs 64 are provided, the discs being uniformaly spaced by spacers 66. The arbor is positioned at an appropriate angle to the tube T so that each of the discs 64 forms a separate distinct helical groove 68, the fins being six in number and also separate and distinct. It will be appreciated that the number of starts which are produced by assembly of finning discs on the tool is determined by the angle at which the arbor is positioned with respect to the axis of the mandrel and tube T. Thus for example, with the six rolls illustrated in FIGURE 5, the same tool could be employed to produce one, two, three, or six-start fin configurations.

During the fin rolling operation which takes place opposite the supporting mandrel 72, the interior surface of the tube is not permitted to be substantially deformed. In order to produce an internal rib 74 of desired height and axial spacing, a separate roll 76 is provided which in this case is positioned with its axis parallel to the axis of the mandrel 72 and tube T. The roll 76 is provided with a helical rib 78 extending at a lead or helix angle determined by the lead or helix angle of the grooves 68 on the tube, and further by the relative diameters or circumferences of the tube and the roll or tool 76. In this case the rib 78 engages in one of the previously formed grooves 68 and deepens it to the increased depth as indicated at 80, simultaneously producing the radially inwardly projecting helical rib 74. Here, where only one of 81X grooves is deepened to produce the rib 74, it will of course be apparent that the pitch or axial spacing between adjacent convolutions of the rib 74 will be six times as great as the pitch or axial spacing between adjacent fin convolutions on the exterior of the tube.

The present invention permits the design of the helical fin configuration on the exterior of the tube to be essen tially independent of the configuration of the internal helical rib. It has previously been indicated that this is desirable because the factors which dictate the most efficient dimensions, shapes, S/H ratios, etc. of the internal rib are quite different and independent from the factors which dictate the most desirable design of finned exterior. It may be mentioned at this time that one con slderation influencing the design of fins at the exterior of the tube is the promotion of drainage of condensate formed on the tube exterior.

The drawings and the foregoing specification constitute a description of the improved configuration of heat transfer tubing for vapor condensation and method and apparatus for manufacturing the same, in such full, clear, concise and exact terms as to enable any person skilled in the art to practice the invention, the scope of which is indicated by the appended claims.

What I claim as my invention is:

1 A metal heat exchange tube for vapor condensation on its exterior surface, said tube having at its exterior surface a plurality of separate generally radially outwardly extending integral helical fins, each fin being formed of material displaced outwardly from material of the tube, said fins being arranged in a multiple start relationship such that adjacent fin convolutions are portions of different fins, separate continuous helical grooves intermediate said fins equal in number to the number of separate fins, the interior surface of said tube having a helically extending cylindrically formed zone occuping a cylinder extending longitudinally of said tube and in registration with at least one helically extending external groove and the two helically extending fins at the sides ofsuch groove, whereby the minimum wall thickness of said tube along said helically extending zone is determined by the spacing between the bottom of the helical groove and the cylindrically formed portion of the interior surface in registration therewith, said interior surface also having a generally radially inwardly extending integral helical rib formed of material displaced inwardly from material of the tube along a helical path in registration with one of said external helical grooves.

2. A tube as defined in claim 1 in which the external groove in registration with said internal rib being deeper than an external groove in registration with the smooth cylindrically formed interior surface, the minimum tube thickness along said internal rib being determined by the minimum spacing between the bottom of the external groove and the surface of the internal rib in registration therewith.

3. A tube as defined in claim 2 in which only a single internal rib is provided in the tube.

6 4. A tube as defined in claim 3 in which only two external fins are provided on the tube.

5. A tube as defined in claim 3 in which more than two external fins are provided on the tube.

References Cited UNITED STATES PATENTS 3,174,319 3/1965 Koyama et a1. 29l57.3 3,213,525 10/1965 Creighton et al 29l57.3 2,244,800 6/1941 Pascale -179 2,463,997 3/1949 Rodgers 165-179 X FOREIGN PATENTS 845,574 8/1960 Great Britain. 540,928 9/1955 Belgium.

ROBERT A. OLEARY, Primary Examiner T. W. STREULE, Assistant Examiner US. Cl. X.R. 29l57.3 

